Intensity.
Intervals.
Going light over a prolonged period of time burns calories BUT it is certainly not the way to go if you're aiming for fat loss. Chances are that you'll just eat those "extra calories you earned".
Stable cardio will elevate your cortisol levels. Cortisol is a hormone which will encourage your body to store fat around your middle section. Not ideal.
High intensity interval training (such as Tabata), on the other hand, helps your body to keep burning body fat even for days after the actual workout. And it's great because you can do it despite your fitness level -- if you're not very fit then you'll just need to do things a bit slower to begin with. As long as you feel like you're genuinely trying hard, that's all that matters.
Because if it's hard, it will work and get you the results you want.
P.s. If you've got medical concerns, please have a catch up with your doctor first to make sure you're fit enough to embark on such a strenuous training protocol.
— and give you the results you want!!